1. Field of the Invention
The present invention relates to a radio terminal that measures its current position with use of radio signals. More particularly, the invention relates to a radio terminal that can calculate a signal receiving point fast and reduce the chip area occupied by semiconductor elements used for the position measurement.
2. Description of Related Art
There have been proposed some techniques related to mobile communication systems; each technique enables respective mobile terminals to detect its current position with use of signals received from base stations in the mobile communication system. For example, JP-A 181242/1995 discloses a technique that measures a position of the subject mobile terminal with use of the position of each base station and a transmission delay time that differs among signals received from those base stations in a CDMA (Code Division Multiple Access) system.
In the case where this mobile terminal receives signals from three base stations, the distance between the mobile terminal and each of those base stations is often different, thereby the signal strength comes to differ among the signals output from those base stations to the mobile terminal. For example, when the mobile terminal positions very closely to a base station, the signal from the base station might be too strong. The signal output from the base station often causes a so-called near-far problem that interferes signals output from other base stations, thereby the receiving quality of the mobile terminal is degraded. If this near-far problem arises, the mobile terminal cannot receive signals from the necessary number of base stations to calculate its current position. In other words, the mobile terminal is disabled to accurately calculate so as to know its current position in an area where the near-far problem has arisen. In order to solve this problem, a method for canceling the interference is proposed, for example, in JP-A No. 166026/2001.
FIG. 12 shows a block diagram of this conventional mobile terminal. A cellular signal received from a base station via an antenna 100 is sent to a signal receiver 101, then subjected to such processings as a receiving processing with a radio/intermediate frequency, base band signal demodulation, A/D conversion, etc. to generate a digital signal. The received signal generated by the signal receiver 101 is stored in a received signal memory 102. The received signal stored in the received signal memory 102 is then inputted to a delay profile generator 103. The delay profile generator 103 generates a delay profile from a received signal stored in the received signal memory 102 with use of, for example, a matched filter. The delay profile generated by the delay profile generator 103 is then written in a delay profile memory 104 and retained there. The delay profile written in the delay profile memory 104 is inputted to both of a position calculator 105 and an interference cancel determination unit 106.
The interference cancel determination unit 106 uses the delay profile stored in the delay profile memory 104 to determine whether to cancel a signal received at a time as an interference one. A received signal replica generator 107 determines an amplitude value of a replica of a received signal with use of the delay profile with respect to the timing at which interference cancellation is required. An adder 108 subtracts the replica of the received signal calculated by the received signal replica generator 107 from the received signal read from the received signal memory 102 to generate an interference-canceled received signal, then writes the signal in the received signal memory 102.
The position calculator 105 calculates the current position of the mobile terminal with use of the interference-canceled delay signals retained in the delay profile memory 104.
In the case of the conventional mobile terminal described above, many calculations are required to generate a delay profile in the delay profile generator 103. For example, in the case of a TIA/EIA/IS-95 system, which is a cellular system that employs the CDMA method, the system needs about 108 calculation circuits. In addition, when consideration is taken to the time to access the received signal memory 102, it takes several tens of seconds to generate a delay profile in the delay profile generator 103. When it takes such a long time to generate a delay profile, the response of position measurement slows down, resulting in the degradation of the serviceability of position information.
The conventional mobile terminal requires a large capacity received signal memory. For example, if the TIA/EIA/IS-95 system has a received signal memory for storing both inphase and quadrature components of received signals for one frame and corresponding to a bit width effective for interference cancellation, the memory needs a capacity of about 106 bits. The received signal memory needs a larger capacity to improve the position measurement accuracy and the quality of received signals. Providing the mobile terminal with such a large capacity memory will cause the mobile terminal to be expanded in size, thereby the portability will be lowered and the manufacturing cost will increase.